As illustrated in Fig. REF , we consider the urban area \(\mathcal {G}\)  on the horizontal plane with the origin \(\mathbf {\rho }_{\mathcal {G}}\)  and containing \(N_0\)  users equipped with an omnidirectional antenna and serviced by stationary UAV-BSs \(\mathbb {M}=\left\lbrace 1,\cdots ,M_0\right\rbrace \) . Additionally, we assume that the UAV-BSs are equipped with a directional antenna where its azimuth and elevation half-power beamwidth (HPBW) are not equal [1]}. The 3D coordinate of UAV-BS \(m\)  is given by \(\mathbf {\rho }_m=\left[\mathbf {w}_m^{\mathrm {T}}~h_m\right]^{\mathrm {T}}\) , which consists of the 2D location \(\mathbf {w}_m\)  and its height \(h_m\) .
